In particle beam radiotherapy, it is important that a sight of a Bragg peak is set right on a tumor as planned. When the position of the tumor fluctuates due to the body motion, etc., a Bragg peak sight will be set at a position different from the planned position. As a method for confirming a sighting point of a Bragg peak, there is a method for detecting, by an electron particle track detecting-type Compton camera, gamma rays generated when particles collide with a living body. However, this is a large-scale, and requires an expensive electron particle track detecting-type Compton camera.